Summary: Feature-rich UI's run the risk of doing too much and being difficult for users to discover. The cure is to remove features and replace them with tasks. Task-features anchor your feature sets in what users want to do, not what you think they want from you.
Popularity is building for a new minimalist smartphone called a 'dumb-phone' aka the Light Phone 2 (pictured above). The phone was deliberately designed without social media apps, to reflect the growing problem of smartphone addiction. The minimalist design strategy of this product offers a good reflection point to consider your 'dumb-phone' strategy for minimalizing feature creep and getting results from Flat Design approaches.